"Your first reaction is to put a stop to it and do what you have to do to put a stop to it," Badger said.

Bill Badger describes what happened during the Tucson shooting.

Daniel Hernandez, an intern who had worked for Rep. Gabrielle Giffords for just a week, also ran toward the gunfire. He held Giffords in his arms and possibly saved her life.

"I was kind of holding her up against my chest and applying pressure to her wound," Herandez said. "I would tell her, 'Gabby, are you still with us? Just grab my hand. Hold tight.'"

Hernandez got a standing ovation in the Arizona legislature Tuesday.

Tuscon's heroes echo cases in the past where ordinary people -- untrained and with very much to lose -- made an instant decision to risk their own lives to save others.

Rohit Deshpande, a professor at Harvard Business School, has delved into the science of heroism to find out what causes someone to spring into action despite the danger to help or save someone else.

In his research, Deshpande focused on how hotel workers took extreme risks to protect guests during the deadly terrorist attacks in Mumbai, India, in 2008.

After several desperate hours of explosions and gunfire, members of the kitchen staff locked arms and formed a cordon around guests as the attackers machine-gunned them down.

In another display of heroism, hotel operators stayed at their phones to call rooms with vital information.

"I was incredibly surprised to hear their stories," Deshpande said.

He found heroism had nothing to do with age, gender or religion. It started with personality.

"It seems that they have a much more highly developed moral compass," he said. "They have this instinct for doing something good for other people. We find this across a whole series of situations. We find people who risk their own lives to protect people from harm."

Heroism Can Come From a Personal Connection, Researcher Says

Acts of heroism also are often about context, such as if the person has a connection to the location where tragedy strikes or not.

For instance, the Mumbai hotel workers considered the Taj Mahal Palace a proud national landmark that they wanted to protect. So they stayed and rescued their guests.

Heroism also can come from the person having a connection to the victims, even if the hero doesn't know them personally.

In Tuscon, Giffords, an elected official, was seen as one of their own by those who sprang into action during the crucial 15 seconds.

Such a "protecting the community" phenomenon has been witnessed before across the United States.

In Panama City, Fla., last month, a security guard took down a gunman who had held members of a school board hostage.

Before the security guard acted, the guman had allowed all the female board members to leave the room, but Ginger Littleton decided to return. She swung her purse at the gunman, trying unsuccessfully to knock his weapon out of his hand.

In Long Beach, Calif., last June, Richard Kemp jumped an armed robber from behind and wrestled him to the ground after the robber held up his bank.